cosπ/9cos2π/9cos3π/9cos4π/9=?
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原式=(sinπ/9cosπ/9cos2π/9cos3π/9cos4π/9)/sinπ/9,
=1/2(sin2π/9cos2π/9cos3π/9cos4π/9)/sinπ/9,
=1/4(sin4π/9cos3π/9cos4π/9)/sinπ/9,
=1/8(sin8π/9cos3π/9)/sinπ/9,
=1/8(sinπ/9cosπ/3)/sinπ/9,
(sin8π/9=sin(π-π/9)
=1/8cosπ/3,
=1/16
=1/2(sin2π/9cos2π/9cos3π/9cos4π/9)/sinπ/9,
=1/4(sin4π/9cos3π/9cos4π/9)/sinπ/9,
=1/8(sin8π/9cos3π/9)/sinπ/9,
=1/8(sinπ/9cosπ/3)/sinπ/9,
(sin8π/9=sin(π-π/9)
=1/8cosπ/3,
=1/16
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